Barrier Island Evolution

1
Barrier Island Evolution
2
Beach Diagram
3
Introduction

• Three main theories of barrier island formation
• Other theories-plate tectonics
• Barrier island migration and long shore current
• Overwash and inlets
• Man-made attempts to stabilize beaches

4
Multiple Theories of Barrier Island Formation

• Three original theories were developed between
  1845-1890.
• Since that time, the theories have been
  re-examined and it has been determined that all
  three theories explain the formation of different
  types of barrier islands
• No one theory is correct and new studies have
  shown that other factors such as plate tectonics,
  sea level, and tidal ranges also have an effect
  on the formation of barrier islands

5
Three main hypotheses for the formation of
barrier islands

• 1845-deBeaumont-Johnson concept of the emergence
  of submarine bars
• 1885-Gilbert-Fisher idea of spit growth and later
  breaching by inlets
• 1890-McGee-Hoyt hypothesis of beach ridge
  submergence

6
Emergence of submarine bars

• 1845-deBeaumont suggested that islands such as
  those found in the North Sea and the Gulf of
  Mexico were formed by what is known as bar
  emergence.
• Submarine bars are located offshore. When waves
  come into the shore, they sweep across the
  shallow bottom and pick up loose sand and deposit
  it on these bars. The bars grow in size and rise
  above sea level to become barrier islands.

7
Emergence Theory

• Waves break over a submerged bar
• Bar emerges above sea level due to deposited
  sediment
• Bar develops into barrier island and lagoon

8
Support of deBeaumont

• 1919-Douglas Johnson reviewed deBeaumonts study
  and supported his hypothesis
• Johnson tested emergence and spit formation by
  drawing profiles across a series of barrier
  islands. He determined that islands were formed
  from submerged bars because there was an area of
  depression in front of the islands. This area of
  depression provided the sediment needed to build
  up the bars.

9
Support of deBeaumont, cont

• He claims that islands formed by submarine bars
  are common on shorelines with a low slope. A low
  slope causes the waves to break away from the
  shoreline and permits the buildup of submerged
  bars off the coast

10
Tests of bar emergence and spit formation

• Island formed by a spit, profile intersection at
  coast
• Island formed from emerged bar, profile
  intersection is on the mainland

11
Against deBeaumont

• If barrier islands were formed from the buildup
  of submerged bars, there should be examples of
  islands in various stages. This is not the case.
  
• Studies by John Hoyt in 1967 of the sediments of
  barrier islands in Texas show no beach or open
  marine sediments on the back side of the island.
  Barrier islands that were formed from bars should
  show evidence of marine fossils.

12
Spit Growth

• 1885-Gilbert proposed that barrier islands were
  formed from spits by currents flowing parallel to
  the coast-the longshore current. Individual
  islands were created when the spits were breached
  by waves during storms, creating inlets.

13
Spit Formation

• Spit starts to grow from point of land
• Spit is extended along the coast by longshore
  currents
• Spit is breached during a storm, forming a tidal
  inlet and barrier island

14
Support of Spit Formation

• 1968-John Fisher agreed with Hoyt that the lack
  of fossils argues against bar emergence. He
  studied the lagoons behind barrier islands and
  concluded if the island was formed from a
  submerged ridge, there should be evidence of
  soils or forests that grew behind the ridge. He
  found no sediments and determined islands must be
  formed from breached spits.

15
Refute of spit formation

• Hoyt agrees that spit formation is on a limited
  scale, in small segments of the coast where there
  is enough sediment supply. He does not believe
  that this theory supports the growth of large
  barrier islands.
• If an island was formed from a spit, the original
  marine beach should be preserved along the
  mainland coast. Hoyt studied the mainland side
  of lagoons and found no evidence of beach
  sediment.

16
Dune-Ridge Submergence

• 1890-W.D. McGee suggested that barrier islands
  were produced by drowning of the coastal area
  during a rise in sea level

17
Beach Ridge Formation

• Dune ridge along the coast
• Rise in sea level starts to submerge dune
• Barrier island and lagoon formed from ridge

18
Support of Submergence

• 1967-Hoyt determined from his studies of
  sediments that islands must be formed by
  submergence of coastal dune fields or beach
  ridges. Once the island was formed, waves and the
  longshore current determined their present-day
  shape

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Other Theories

• 1971-Maurice Schwartz published a compilation to
  draw the theories together to show that there are
  multiple origins for barrier islands. He stated
  that there are basically two types of islands
• Primary-islands that were formed on land that is
  later flooded by the sea
• Submergence
• Secondary-islands that developed seaward of the
  primary coast on the continental shelf
• Spits and emergence

20
Role of Tectonics in Formation

• Three types of tectonic settings throughout the
  world
• Trailing Margins
• Collision Margins
• Marginal Seas

21

• Collision Coast Trailing Edge Coast
• Blue-Continental Yellow-Neo-trailing
• Red-Island Arc Pink-Afro-trailing
• Green-Amero-trailing
• Ligt Blue-Marginal Sea Coast

22
Percentages of Islands along Coastlines

• Trailing Margins 49
• Atlantic Seaboard
• Collision Margins 24
• Western Central America
• Marginal Sea Coasts 27
• Baja California

23
Additional Factors

• Three additional factors contributing to
  formation of barrier islands
• Continental shelf width
• Sediment supply
• Tectonic setting
• Majority of islands are found on trailing margins
  because all three factors are favorable
• However, islands are found on collision and
  marginal sea coasts because they have a wide
  coastal plain even though they do not have a
  wide, flat continental shelf or a history of
  sediment supply

24
Coastal Zone
25
Barrier Island Migration

• The beach system includes the area from the dunes
  to 30 feet deep offshore. The sand in this
  system is constantly moving.
• Evolution of barrier islands includes
• Sand bars moving up to the beach
• Beaches flattening and sand moving offshore
• Islands retreating toward the mainland
• Islands migrating along the coast
• Islands prograding toward the ocean
• Inlets opening, closing, or migrating

26
Movement on the Beaches

• Sand is constantly moving due to wind, waves and
  currents
• During fair weather, gentle waves cause a gradual
  movement of sand bars toward the mainland.
• Net movement of sand is onshore
• During storms, powerful wind and waves erode the
  shorelines
• Storm surges and waves may pass through breaks in
  dunes and deposit sand in washover fans behind
  the dunes
• Storms can pull the sand offshore and flatten out
  the beach
• Net movement of sand is offshore

27
Longshore Current

• Waves typically strike a beach at an angle,
  causing a current to run parallel to the coast
• In NC, the longshore current runs from North to
  South
• Longshore current allows sediment to be carried
  down the beach.

28
Longshore Current
29
Movement landward

• Transgression is the movement of islands landward
• Rate of sea level rise overcomes the amount
  sediment supply
• Transgression is caused by waves carrying
  sediment from the beaches and dunes to the back
  of the barrier island and depositing it in
  washover fans.
• The natural movement of a barrier island is to
  migrate landward as sea level rises.
• Front side of the island is eroded by wave action
• Back side must grows from the buildup of washover

30
Coastline Moving Landward
31
Evidence of Transgression

• Studies of sediment on barrier islands show peat,
  tree stumps, oyster shells and layers of mud on
  the beach side
• Once were part of the marsh, now found on the
  beach
• Marsh has a floor of thick, black mud from the
  ocean side
• As the island rolls back, the beach moves back
  over the old marsh. Todays beach is where the
  marsh used to be.
• Island rolls over on itself like a conveyor belt

32
Movement Seaward

• Progradation is the movement of islands seaward
• Process is different from transgression because
  the island itself does not move
• Sediment is pushed on shore, building up the
  ocean side of the island
• The addition of sediment causes the development
  of multiple beach-dune systems
• Shoreline on the ocean side moves seaward while
  the landward back side stays stationary
• Brought about by an excess of sediment that can
  not be carried away by the longshore current

33
Coastline Moving Seaward
34
Barrier Island Changes

• Barrier islands can experience both transgression
  and progradation at the same time or change back
  and forth over time
• Barrier islands may change the shape of the
  shoreline, but generally maintain their position

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Shoreline Changes
36
Coastline Changes
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Hog Island Changes
38
Changes to Hog Island Over the Past 350 Years
39
Movement Parallel to the Coast

• Barrier islands may experience erosion on one end
  and a build-up of sediment on the other end
• Migration of inlets can contribute to changes in
  shape and size of barrier islands

40
Effects of Inlets

• Inlets are important because the are a site of
  storage and transportation of sand
• Sand being carried by the longshore current gets
  carried into tidal inlets
• Inlets move in three general ways

41
Shifting Inlets

• Breathing Inlets expand and contract
• Migrating inlets form in one place and migrate
  with the longshore current
• Returning inlets migrate, but may return to their
  original position

42
Masons Inlet
43
Shell Island

• October 1989  May 1990 Average Distance
  Change180ft/0.6 yr

44
Shell Island

• November 1993 November1995
• Average Distance Change Average Distance
  Change
• 670 ft/3.6 yrs
  440 ft/1.9 yrs

45
Shell Island 1994-1998

46
Inlet after Hurricane Hugo-SC
47
Tidal Deltas

• There are two types of deltas
• Ebb tidal delta develops on the ocean side of the
  island
• Flood tidal delta develops on the lagoon side of
  the island protected from effects of waves
• Built by the sediment deposition that accumulates
  when a current suddenly slows at the mouth of a
  channel, losing its carrying capacity

48
Deltas, Inlets, Islands
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On-line References

• http//books.nap.edu/books/030904806/html/70.html
  pagetop
• http//coastal.er.usgs.gov/hurricanes/dennis/buxto
  n/
• http//members.nbci.com/igorz/MIWO.html
• http//www.beachbrowser.com/Archives/Environment/A
  /Barrier-Islands-Always-changing.ht
• http//www.csc.noaa.gov/products/nchaz/htm/lidtopo
  .htmhttp//www.lacoast.gov/Programs/CWPPRA/Waterma
  rks/Fall99/barrier.fa99.htm

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References

• http//www.ncsu.edu/coast/shell/migration/migratio
  n.html
• http//www.ncsu.edu/coast/shell/terms.html
• http//www.salem.mass.edu/lhanson/gls214/gls214_t
  ec_clas.htm
• http//www.seafriends.org.nz/oceano/beacheng.htms
  ea20walls
• http//www.seashell.com/kbphotos.htm
• http//www.vcrlter.virginia.edu/bph/NSFTalk/NSFTa
  lk.html

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References

• Davis, Richard, 1994, The Evolving Coast W.H.
  Freeman and Company, New York, pp 166-183.
• Fox, William, 1983. At the Seas Edge,
  Prentice-Hall, Inc., pp. 151-175.
• Pilkey, Orrin, 1990. The Beaches are moving
  (videorecording).
• Schwartz, Maurice, 1973, Benchmark Papers in
  Geology Barrier Islands Dowden, Hutchinson and
  Ross, Inc., Stroudsburg, Pa.